1. Field of the Invention
The invention relates to a tuner and to a method for correcting the alignment of a phase-locked-loop demodulating stage in a tuner.
2. Description of the Prior Art
A known form of tuner for use within an analogue TV satellite receiver is illustrated in FIG. 1 of the accompanying drawings as reference designator 10 and comprises an AGC (automatic gain control) filter stage 12, a downconverter and SAW-filter stage 13 fed from the AGC filter stage, a frequency synthesiser stage 14 connected so as to control the frequency of the local oscillator 15 in the downconverter stage, and a PLL (phase-locked loop) FM demodulator stage 16 fed from the output of the downconverter stage 13. A microcontroller 17 controls the various functions of the tuner, including setting up the synthesiser to tune the local oscillator 15 to the requisite frequency for the selection of the RF channel of interest, and receiving an AFC (automatic frequency control) flag signal 18 from the PLL stage for purposes to be described later. Preceding the tuner 10 is a head unit 20 connected to a satellite dish 21 and containing a down-converter and low-noise amplifier stage 22, which feeds the RF input of the tuner 10.
The PLL block 16 contains a reference-signal generator in the form of a voltage-controlled oscillator (VCO) 23 which generates a signal whose frequency is compared with the incoming signal from the downconverter stage 13 in a phase comparator 24. Control of the VCO frequency is achieved by, for example, the use of a varactor diode 25, the capacitance of which is varied from the output of the phase comparator 24 by way of an amplifier 26. The varactor diode 25 is shunted by a reactance, in this case the inductor 27, for reasons to be explained shortly.
There are three centre-frequencies associated with the tuner 10. These are: the centre-frequency of the IF (intermediate frequency) following down-conversion in the down-converter stage 13, this centre-frequency being a function of the frequency accuracy of the head-unit down-conversion stage 22 and the frequency-accuracy of the local oscillator 15; the centre-frequency of the SAW filter 19 in the down-converter stage 13, and the centre-frequency of the VCO in the PLL stage 16. In the interest of providing optimum signal-to-noise ratio, it is desirable that all three of these coincide as far as possible. This is shown diagrammatically in FIG. 2 of the accompanying drawings, in which curve 30 represents the bandwidth of the SAW filter 19, curve 31 the frequency characteristic of the IF entering the SAW filter 19 and curve 32 is the controllable frequency range of the VCO in the PLL stage 16. In the ideal state, the centre of the VCO""s frequency range, the centre of the IF and the centre of the SAW""s response should coincide at 33.
In the known alignment procedure two alignment processes are carried out: one manual and mechanical and performed during alignment of the tuner in production, the other automatic and electronic and performed during normal operation of the tuner. The manual/mechanical alignment procedure generally consists in injecting a signal of a known carrier frequency into the tuner, allowing this signal to be downconverted to IF by the downconverter stage 13, then adjusting the physical dimensions of an air coil of the inductor 27 to centre the VCO output frequency onto the IF frequency. The alignment is monitored either by measuring the video output signal, or by sampling the AFC data generated by the PLL demodulator on line 18. In a typical set-up involving the first of these monitoring alternatives, a signal of, say, 1.5 GHz frequency is input into the AGC section 12 modulated with, say, a xc2x120 MHz ramp signal. Two signals will then be fed into the X- and Y-terminals of an oscilloscope: the ramp modulating signal into the X-terminals and the PLL-demodulated output signal (xe2x80x9cVideo Outputxe2x80x9d) into the Y-terminals. The air core of the inductor 27 is then adjusted until the appropriate waveform is obtained on the screen, at which point the VCO is correctly aligned.
The automatic alignment involves the AFC function already mentioned. This function compares the centre-frequency of the VCO to the IF centre-frequency at the output of the SAW filter and, if the latter is higher than the former, the output on line 18 goes, say, logic low (xe2x80x9c0xe2x80x9d), whereas if the IF centre-frequency is lower than the VCO centre-frequency the AFC output on line 18 goes high (xe2x80x9c1xe2x80x9d). This is illustrated in FIG. 3 of the accompanying drawings, in which xcex94F is the difference in the centre-frequencies. The logic signal on line 18 is used by the micro-controller 17 to adjust the frequency of the downconverter local oscillator 15 (via the synthesiser 14) in a direction such as to bring the two centre-frequencies into alignment.
Use of an AFC function in this way corrects for errors in the centre-frequency of the head unit 20 due mainly to the susceptibility of the unit to temperature fluctuations. These can be quite severe, since the unit is usually situated in the open air and may therefore be subject to extreme weather conditions.
The afore-mentioned inductor adjustment is a procedure which conventionally takes place on the bench in a production-line setting and is, by its very nature, quite cumbersome and expensive, since it requires either costly manual labour to perform it, as described above, or some kind of test machine. By way of example, if the alignment costs $40 per hour and three units are aligned per minute, then the alignment cost per unit is $0.22. This known alignment procedure is not just expensive, however, it is also far from perfect. This is because it does not compensate for drift in the frequencies of the local oscillators 15 and 23 which can arise due to ageing, temperature fluctuations (not so serious as in the head unit) or the use of a different tuner supply voltage form the voltage applied during adjustment of the inductor 27. Also, if the centre-frequency of the VCO 23, specifically, is in error, the AFC function will assume this is a head-unit error and so wilt xe2x80x9ccorrectxe2x80x9d for it by incorrectly aligning the tuner. The result, in an analogue TV system, is a compromised performance with visual impairments to the picture being apparent to the viewer.
There is therefore a need for an alignment arrangement which takes into account the effects of factors such as ageing and temperature and supply-voltage variations in the tuner and does so, ideally, in a more cost-effective manner than the known alignment arrangement. It would also be advantageous if such a new arrangement were to involve minimal redesign of the known arrangement, especially as far as hardware is concerned.
In accordance with the present invention, there is provided a tuner comprising an RF input stage, a downconverter stage fed from said RF input stage for providing an if signal, a phase-locked-loop demodulator stage fed from said downconverter stage and comprising a reference-signal generator, characterised in that said tuner comprises an alignment arrangement for use during an alignment phase of said tuner, said alignment arrangement comprising: means for injecting into said demodulator stage during said alignment phase an alignment signal having a frequency corresponding to a nominal value of a centre-frequency of said IF signal; means for adjusting the frequency of said reference signal until it is substantially aligned with the frequency of said alignment signal; and means for thereupon removing said alignment signal.
The reference-signal generator may be a voltage-controlled generator having a control input, and the means for adjusting the reference-signal frequency may comprise a variable voltage means connected to the control input. The variable voltage means may be a digital-to-analogue converter.
The tuner may comprise an automatic frequency-control (AFC) arrangement configured to maintain, during normal use of the tuner, substantial agreement between the IF centre-frequency and a centre-frequency of the reference-signal generator. The AFC arrangement may generate a flag signal for the indication of said agreement and the means for adjusting the frequency of the reference signal may make use of the flag signal to determine when the reference-signal frequency is substantially aligned with the alignment-signal frequency. The flag signal may derive information concerning said agreement from the demodulator stage.
The downconverter stage may comprise a local oscillator controlled from a frequency synthesiser, and the injecting means may comprise a prescaler output of the frequency synthesiser. Alternatively, the local oscillator may feed a divide-by-four stage which, in turn, supplies a change-over switch means, the output of which feeds the demodulator stage. The tuner may be arranged to control the local oscillator such that it feeds a signal at four times the nominal IF centre-frequency to the divide-by-four stage, and may activate the switch means such that an output of the divide-by-four stage is routed through to the demodulator stage during the alignment phase.
The tuner may comprise a means for disabling normal IF operation during the alignment phase.
In a second aspect of the invention, there is provided a method for correcting the frequency alignment of a reference-signal generator forming part of a phase-locked-loop demodulator stage in an analogue tuner, said tuner comprising an RF input stage and a downconverter stage fed from said RF input stage and connected to said demodulator stage for the feeding of an IF signal thereto, said method comprising the steps of, in an alignment phase: injecting into said demodulator stage an alignment signal having a frequency corresponding to a nominal value of a centre-frequency of said IF signal and adjusting the frequency of said reference signal until said alignment-signal frequency and said reference-signal frequency are in substantial alignment; then, to enable a normal operation of said tuner, removing said alignment signal.